In the rotary drilling of wells a drilling fluid circulates throughout the underground well to carry cuttings from the bit and to transport these cuttings to the surface. Contemporaneously, the drilling fluid cools and cleans the drill bit, as well as reduces friction between drill string and the drilled hole, and also stabilizes uncased sections of the well. Usually drilling fluids form a low permeability filter cake in order to seal any permeability associated with the surrounding geological formations.
Drilling fluids may be classified according to their fluid base: oil based fluids with solid particles suspended in an oil continuous phase and, possibly, water or brine may be emulsified with the oil. Alternatively, water based fluids contain solid particles suspended in water or brine. Various solids may be added, deliberately or otherwise, to water based drilling fluids: a) organic polymers or clays used to impart viscosity and filtration properties; b) insoluble inorganic minerals to increase the fluid density as well as help decrease fluid loss; c) soluble salts used to increase the mud's density; and d) during the drilling operation formation solids may disperse into the drilling fluid.
Formation solids that become dispersed in a drilling fluid include cuttings from drilling, soil, and solids from surrounding unstable formation. When the formation yields solids that are clay minerals which are reactive, for example swell, disperse, migrate or undergo swelling-induced migration, this can potentially compromise drilling time and increase costs.
Clays are typically composed of sheets or layers of aluminosilicate minerals having exposed surface hydroxyls. The basal plane of the clay surface is negatively charged and as such cations are readily adsorbed onto the surface. These cations may be exchangeable. Substitutions within the clay structure and the presence of exchangeable cations affect the tendency of the clay to swell in water. For example surface hydration gives swelling with water molecules adsorbed on clay surfaces. Many types of clays can swell in this manner.
Another type of swelling is called osmotic swelling, when interlayer ion concentration leaches water between the clay unit layers, swelling the clay. Only some clays can undergo osmotic swelling. All types of clay and shale instability, such as swelling, can cause a series of problems. For example, drag between the drill string and the sides of the borehole may be increased. This can cause loss of fluid circulation and sticking of the drill string and bit.
This is why development of effective clay instability inhibitors is important to the oil and gas exploration industry. The present invention works towards a solution to these difficulties.
Many types of clay inhibitors are known including the use of inorganic salts such as potassium chloride. Numerous patents have been filed which describe techniques or products which can be used to inhibit clay swelling. Without completely summarizing the patent literature, and by way of example, we can cite the inhibitor compositions based on: a) inorganic phosphates, described in U.S. Pat. No. 4,605,068; b) polyalkoxy diamines and their salts, in U.S. Pat. Nos. 6,484,821; 6,609,578; 6,247,543; and U.S. 20030106718; c) choline derivatives described in U.S. Pat. No. 5,908,814; d) oligomethylene diamines and their salts, in U.S. Pat. No. 5,771,971 and U.S. Publication No. 20020155956; e) the addition product of carboxymethyl cellulose and an organic amine, in WO 2006/013595; f) 1,2-cyclohexanediamine and/or their salts, in WO 2006/013597; g) salts of phosphoric acid esters of oxyalkylated polyols, in WO 2006/013596; h) the combination of a partially hydrolyzed acrylic copolymer, potassium chloride and polyanionic cellulose, in U.S. Pat. No. 4,664,818; i) quaternary ammonium compounds, in U.S. Pat. Nos. 5,197,544 and 5,380,706; j) polymers based on dialkyl aminoalkyl methacrylate, in U.S. Pat. No. 7,091,159; k) aqueous solutions containing a polymer with hydrophilic and hydrophobic groups, in U.S. Pat. No. 5,728,653; and l) the reaction product of a polyhydroxyalkane and an alkylene oxide, in U.S. Pat. No. 6,544,933.